1. Field of the Invention
The present invention relates to an image signal transmission system, and more particularly to a system capable of transmitting and receiving hierarchically encoded image signal through a transmission line (satellite, ground wave, cable etc.) or a recording medium.
2. Related Background Art
Recent progress in the semiconductor technology has stimulated the digitization of the television image, thus enabling digital transmission in the broadcasting and in the storage medium. However, the digital transmission, having steep threshold characteristics at the practical limit, is associated with a drawback that the image quality becomes deteriorated suddenly beyond the limit point. For example in a satellite system generally utilizing the ku band (14/12 GHz) or the ka band (30/20 GHz) for the carrier frequency, there is encountered significant attenuation by rain, and the C/N ratio of the received signal becomes extremely low in a hard rain condition. Also in a ground wave system, the signal level is lowered behind a building, in case of reception in a moving vehicle. A similar phenomenon is experienced also in case of signal reproduction on a video cassette recorder or the like, caused for example by head clogging.
In order to overcome such drawbacks mentioned above, there has been developed the hierarchic encoding-decoding technology capable of so-called "graceful degradation", which, under an unfavorable receiving or reproducing condition, preferentially reproduces the information important in the image configuration thereby enabling recognition of the content of the image, while disregarding the information of lower priority.
In such "graceful degradation", a low-quality image signal obtained by compressing the input image signal with a very high compression rate and a residual image signal obtained by encoding a difference (residual) signal, representing the difference between a signal decoded from the above-mentioned low-quality image signal and the original signal, are transmitted in multiplexed form.
The above-mentioned low-quality image signal is of low hierarchic quality allowing enough recognition of the content of image by itself, and a high-quality image of higher definition can be obtained at the reception side, by adding the residual image signal to such low-quality image signal.
FIG. 8 shows the encoding characteristics of two transmission lines applied in the hierarchic encoding-decoding technology mentioned above, with the theoretical values in case of QPSK digital modulation for the transmission. The abscissa indicates the C/N ratio of the transmission line, while the ordinate indicates the bit error rate (BER: rate of error per unit amount of information in the transmitted bit stream). In FIG. 8, a curve A represents the QPSK characteristics without encoding, while a curve B indicates the transmission characteristics after a convolution encoding of r=7/8 on the remainder bit stream, and a curve C indicates those after a convolution encoding of r=1/2 on the low-quality bit stream.
As shown in FIG. 8, a satisfactorily low bit error rate (BER) corresponding to sufficient image quality can be obtained both in the low-quality bit stream or the residual bit stream, if the C/N ratio is sufficiently high. However, when the C/N ratio is lowered to about 8 dB, the BER of the residual bit stream represented by the curve B reaches a level of 1.times.10.sup.-5 which is insufficient for the error correction, but the low-quality bit stream represented by the curve C still provides a sufficiently low BER. When the C/N ratio is further lowered to about 6 dB, the BER of the residual bit stream of the curve B approaches 1.times.10.sup.-2, so that most of the received data become error. Even in such state, the low-quality bit stream of the curve C shows a BER as low as about 1.times.10.sup.-10. When the C/N ratio is further lowered to 4 dB, the amount of errors becomes higher even in the low-quality bit stream.
Since the BER of the residual bit stream varies significantly in a threshold range of the C/N ratio around 6 to 8 dB, if the bit stream fluctuates around or remains in such threshold range, the quality of the decoded image signal varies frequently between the upper hierarchic quality and the lower hierarchic quality, thus creating a very unpleasant situation for the user.